1. Field of the Invention
The present invention relates to a semiconductor device fabrication method and, more particularly, to a technique of cutting and dividing a semiconductor wafer having undergone element formation into semiconductor chips.
2. Description of the Related Art
Conventionally, a mechanical method such as using a dicing blade or scriber forms cutting grooves along dicing lines or chip dividing lines of a semiconductor wafer having undergone element formation, and a breaking method or expanding method divides the wafer (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-198326). It is also possible to use a method which forms modified layers (brittle layers) by irradiating a wafer along chip dividing lines with a laser or the like, thereby dividing the wafer (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2004-79746).
When the breaking method or expanding method divides a semiconductor wafer, however, the starting point of breakage (cleavage) of the wafer often becomes unstable, allowing the cleavage plane to reach an interconnection region formed inside a chip, thereby producing defective products. This is so because the starting point of cleavage is, e.g., a flaw, strain, or crystal defect produced when a dicing blade or scriber mechanically forms a cutting groove. Also, even when cleavage does not reach the interconnection region and hence has no influence on an internal circuit, variations in chip shape or undulation of the edge pose problems in later steps, thereby decreasing the fabrication yield.
On the other hand, the method which forms modified layers by laser irradiation or the like can avoid the problems of the mechanical method as described above. However, a distance (depth) in the direction of thickness of a chip which can be modified by laser irradiation is presently at most about 20 to 30 μm. To divide a wafer by laser irradiation, it is necessary to modify a distance equal to or larger than the final thickness (the cleavage plane on a side surface) of a semiconductor chip, and equal to or larger than about 50% of the thickness of a wafer when it is divided. For example, when the thickness of a wafer when it is divided is 500 μm and the thickness of a chip when it is finally encapsulated into a package is 100 μm, it is necessary to form a modified layer having a thickness of at least 300 to 400 μm (inclusive). Therefore, a modified layer must be formed by irradiating the same line with a laser about 10 to 13 times while the focal position in the direction of depth (the formation position of the modified layer) is changed. This prolongs the time necessary before cutting, and worsens the productivity.
Also, the back surface of a semiconductor wafer having a diameter of 8 inches or less is a satinized surface. When a laser irradiates the back surface, therefore, no modified layer is formed inside the wafer because the roughened surface irregularly reflects the laser. Accordingly, the back surface of the semiconductor wafer must be planarized to a plane surface having a grit of #2000 or more by grinding or etching. This reduces productivity.